Hepatitis E (HEV) is enterically transmitted RNA virus that causes acute viral hepatitis (AVH) in many lesser developed countries (LDC), with frequent reports of fulminant hepatitis in pregnant women. Although HEV- caused AVH is not endemic in the US, the prevalence of antibodies to HEV (anti-HEV) is as high as 20% among blood donors in certain areas of the US. HEV is a zoonotic disease with several animal species being reservoir hosts. The four genotypes of HEV are cross-reactive serologically but have different geographical distributions, species specificity, and virulence. However, cross-species infections have been reported. In the Indian subcontinent, and recently in outbreaks in Baghdad and in refugees in Darfur Sudan, the virulent genotype-1 strain of HEV was isolated from AVH cases. We reported HEV is endemic in rural villages in Egypt. In spite of community-wide anti-HEV prevalence of 70-85%, HEV-caused AVH was rare and fulminant HEV-hepatitis was not detected among pregnant women. However, HEV genotype-1 has been isolated from the stool of two hospitalized AVH patients in Cairo; and HEV has accounted for 10-to-30% of AVH in hospitalized Egyptians. We have recently isolated avirulent genotype-3 HEV (the same strain found in the US) from domestic animals in Egypt. Our exploratory development award's purpose is to prepare, and obtain preliminary data, to test hypotheses that may explain why HEV-caused AVH may be rare in some areas, like Egypt and the US, despite the presence of high anti-HEV prevalence. Among these are: (1) Prior early exposures to avirulent strains of HEV lead to asymptomatic or mild infections, prime immunity, and suppress viremia and clinical manifestations during subsequent infections with more virulent strains. (2) Two HEV genotypes are transmitted in Egypt: (a) genotype 3 is endemic and zoonotically transmitted among animal reservoirs causing avirulent infections in humans; and (b) the virulent genotype-1 HEV sporadically causes AVH. Cell-mediated immune (CMI) testing is necessary to test our hypotheses; and HEV-specific CMI responses may be a long-lasting reliable marker of prior exposure and/or protection to subsequent infection/morbidity. There are no reported HEV CMI studies, and the serological cross-reactivity among HEV strains, limits utilization of anti-HEV as surrogate markers for prior exposure to avirulent strains of HEV. Therefore, we plan to develop, standardize and use reliable tests to measure CMI responses against unique epitopes for HEV strains. This will provide the means to sort-out the different roles the virus and the host's immune response have on transmission and morbidity of HEV in Egypt, and the results would be applicable to other areas, including the US. Consequently, this application will provide the means and preliminary data to design a protocol to understand the factors involved in the pathogenesis of HEV, as one of the important emerging infectious diseases that appears to have spread from animals to man. [unreadable] [unreadable] [unreadable]